1. Field of the Invention
This invention relates generally to welding methods and products, and more specifically to methods for repair of single crystal superalloys by laser welding and products thereof.
2. Description of the Related Art
In the past, nickel-based superalloys have been widely used for applications in which resistance to high temperatures is required. Such applications are largely found in the hot-section parts of gas turbine engines. Improvements in manufacturing methods have led to casting of components in single crystal form giving better high temperature lives and strength than their equiaxed, poly-crystalline counterparts. Over time, such components (particularly turbine blades) would be damaged or degraded due to erosion, corrosion, oxidation and thermal fatigue, and require repair or restoration. Amongst other causes were the fuel gas used in power generation gas turbines which contain many compounds such as sodium sulfate and sodium chloride, etc., which promote high temperature corrosion and oxidation of turbine blades.
The efficiency of gas turbine engines depends in part on the ability to minimize leakage of compressed air between the turbine blades and the shrouds of the engine's turbine section. During service operation, as noted above, turbine blade tips and vane air foils are eroded and worn due to severe oxidation, corrosion and foreign particle impact, and such damaged components become scrapped parts. It is therefore advantageous to repair components if at all possible.
However, because these components comprise superalloys having high strength, but low ductility properties, cracks would be induced during welding. The cracks would propagate rapidly under stress resulting in unusable parts. Superalloys are principally strengthened through controlled heat treatment producing Ni3(Al, Ti) precipitates known as gamma-prime. The precipitation hardening phenomena and the associated volumetric changes that occur upon aging facilitates cracking and makes welding of these alloys very difficult.
As a result of the high temperatures involved in welding, there is a need for an improved process permitting superalloy parts to be joined with other parts of similar alloy composition while maintaining the strength and ductility properties of both the parts and the integrity of the bond that is thereby formed. An improved process for coating existing parts that have corroded, as discussed above, is also desirable.
For the foregoing reasons, it is desirable to provide improved methods for repair of single crystal superalloys by laser welding and products thereof.